A Flow Graph Technique for DFT Controller Modification Mohammad Hosseinabady, Pejman Lotfi-Kamran, Pedram Riahi*, Fabrizio Lombardi*, Zainalabedin Navabi Electrical and Computer Engineering, University of Tehran, Iran *ECE Department, Northeastern University, USA

Agenda Motivation, Objectives and Contributions Preliminaries and Our Idea CDFG Testability and Victim An Example Experimental Results

Agenda Motivation, Objectives and Contributions Preliminaries and Our Idea CDFG Testability and Victim An Example Experimental Results

Motivation and Objectives Test generation at the gate-level produces high-quality tests But in the case of large systems It is computationally expensive Objectives We propose a DFT algorithm to Reducing test generation time Reducing test application time

Our Contributions Realize and utilize existing path of the datapath by changing controller Previous work Increase the testability of an RTL module Use datapath as search space to find test paths But, we Increase testability; decrease test application time. Use the CDFG as search space to find test paths

Advantages of Our Algorithm Reduce test application time Using pre-computed test vector Reducing test generation effort Very small area overhead on controller No timing penalty of critical path (in the datapath)

Agenda Motivation, Objectives and Contributions Preliminaries and Our Idea CDFG Testability and Victim An Example Experimental Results

CDFG and Synthesis CDFG can be used for extracting control The behavioral description of a circuit is usually provided using a hardware description language like VHDL. This description is compiled into a control/data flow graph (CDFG), which is a directed graph with operation vertices, data variable arcs, conditions, and loops. CDFG can be used for extracting control and data information during synthesis steps Scheduling Binding CLK1 *1 *1 +2 *3 z2 z1 z3 CLK2 *4 +5 3 z4 z6 *6 *7 CLK3 5 z7 CLK4 z5 *8 z8 CLK5 -9 y

Datapath & Controller - + * z1,z4,z6,y z3,z7,z8 z2,z5 m1 m2 m3 m4 L1 b c d f g 1 2 m5 m6 m7 L3 e 3 5 Group 1 Group 3 Group 2 Group 4 R1 R2 R3

Test-Path in the Datapath Module Under Test (Pre-computed test vectors are available) g a f 5 c e d b 3 * * + - Group 2 Group 3 Group 1 Group 4 z3, z7, z8 z2, z5 z1, z4, z6, y

A Problem!!! This Test-Path is not supported by the controller!!! Using the CDFG as the search space, a controller supported Test-Path can be found

Agenda Motivation, Objectives and Contributions Preliminaries and Our Idea CDFG Testability and Victim An Example Experimental Results

Test-Path in CDFG The role of a controller is to map the CDFG of a circuit to its datapath. Consequently, All paths in a CDFG exist in the datapath of a circuit. However, there may be additional paths in a datapath that do not necessarily exist in a CDFG that the datapath is generated from.

Test-Path in CDFG controller-supported paths: paths in a datapath can be activated for passing data by the existing controller of the circuit. controller-unsupported paths : paths of the datapath that are not specified by the CDFG and are not activated by the existing controller To recognize and thus utilize these paths, changes may be needed in the circuit’s controller.

Faulty Operator in the CDFG If a module in the RTL is faulty, all operators in its group of the CDFG became faulty. This means that multiple faults appear in the CDFG. To avoid this scenario, only one operator is considered as a victim operator as faulty and the CDFG is pruned by deleting all other operators in the group as well as all their connecting nodes.

Victim A victim is an existing/new operator that Receives its inputs from outside of its group and Propagates its output to outside of its group, and Pruned CDFG corresponding to the victim operator must have at least a primary output of the original CDFG.

Victims Victim Victim a b Victim b a a a a b b b Group i c Group i

Test Time Reduction Test application time can be reduced using two techniques Finding best victim when a victim has alternatives Removing unnecessary states in the Pruned CDFG (Explained in the example)

Agenda Motivation, Objectives and Contributions Preliminaries and Our Idea CDFG Testability and Victim An Example Experimental Results

Group 1 * * + - g a f 5 c e d b 3 Group 1 Group 2 Group 3 Group 4 z3, z7, z8 z2, z5 z1, z4, z6, y

z1 and y are mapped in the same register in the Datapath Group 1 Victim e g a b c d f Victim CLK1 *1 *1 +2 *3 z2 z1 z3 z1 and y are mapped in the same register in the Datapath CLK2 *4 +5 3 Group 1 z4 z6 *6 *7 CLK3 5 z7 CLK4 z5 *8 z8 CLK5 -9 y

Group1 Testing Pruned CDFG Just one clock cycle is enough a b Just one clock cycle is enough to apply each pre-computed test vector to this RTL module CLK1 *1 *1 y Pruned CDFG

Group2 * * + - g a f 5 c e d b 3 Group 1 Group 2 Group 3 Group 4 z3, z7, z8 z2, z5 z1, z4, z6, y

Group 2 Victim Victim e g a b c d f *1 +2 *3 Group 2 *v z2 z1 z3 z1 and z6 are mapped in the same register in the Datapath *4 +5 3 z4 z5 *6 *7 5 z7 z6 *8 z8 -9 z3 and z8 are mapped in the same register in the Datapath y S1 S2 S3 S4 S5

Group 2 Testing a b g f Just two clock cycles are enough to apply each pre-computed test vector to this RTL module CLK1 *1 *3 CLK5 -9 y S1 S2 S3 S4 S5

Group4 * * + - g a f 5 c e d b 3 Group 1 Group 2 Group 3 Group 4 z3, z7, z8 z2, z5 z1, z4, z6, y

Group4 Victim Group 4 has only one member, so it is a compulsory victim operator. The DFG for this victim is the same as the original CDFG . This CDFG can be reduced by the elimination of clocks 2, 3 and 4. e g a b c d f CLK1 *1 *1 +2 *3 z2 z1 z3 CLK2 *4 +5 3 z4 z5 *6 *7 CLK3 5 z7 CLK4 z6 *8 z8 CLK5 -9 Group 4 Victim y

Reducing the CDFG Graph of Group 4 b c d f The utilization of shared registers of the datapath are the basic principle for reducing the test application time. Using a bipartite graph the obtained CDFG is reduced *1 *1 +2 *3 z2 z1 z3 *4 +5 3 z4 z5 *6 *7 5 z7 z6 *8 z8 -9 y

Group 4 Testing a b g f Just two clock cycles are enough to apply each pre-computed test vector to this RTL module CLK1 *1 *3 CLK5 -9 y S1 S2 S3 S4 S5

Agenda Motivation, Objectives and Contributions Preliminaries and Our Idea CDFG Testability and Victim An Example Experimental Results

Experimental Results

Questions!